This is a multi-faceted question and my answer would be: it depends. Generally, the one that you like enough, and that is comfortable enough, that you will actually use it. One of the best pieces of equipment for a hard workout and a high-calorie burn is a Stepmill. The problem is most people can only last on it for 10 or so minutes, or if they stay on it any longer they are leaning on the hand rails and using terrible form or moving so slowly that the benefit is minimal. If you can really stand upright, use your abdominals and other core muscles to support yourself in this position, and also lift your legs up as if you’re climbing rather than just pressing on the stairs, it can be an amazing workout. If you can also add some high speed intervals, but at a level where you are still able to maintain the same good form, it becomes one of the best cardiovascular options out there. In general, however, you can get a great (hard) workout on almost any machine, provided you are using it correctly. The question is: does it allow you to really push yourself?
But to better answer the question of “best” piece of cardiovascular equipment properly you’d have to ask yourself: What position do I spend most of the day in? What aches and pains do I have? What hurts?If you sit all day at work and spend a considerable amount of time in the car, you probably shouldn’t use a bike, but rather a piece of equipment that will force you to work on your posture and stretch the muscles that get tight while sitting. If, on the other hand, you have knee problems, a bike would definitely be easier on your joints. If you’re very overweight but would like to do something standing, then an elliptical trainer is probably ideal. Elliptical trainer will put a bit more pressure on the knees than a bike so that’s why sometimes people find walking on the treadmill easier than being on an elliptical trainer. If you’re overweight AND have knee problems, you’ll have to make a smart choice between a bike and a treadmill to choose what’s gentler on the joints AND what gives you a better overall workout.
If you want to increase the challenge and walk on the treadmill uphill, make sure that your hip flexors are stretched enough to allow you to keep the upper body upright, instead of bent over and with your butt sticking out a mile behind you.
So, to sum it all up: the best piece of cardiovascular equipment, or best cardiovascular exercise, is the one you like and therefore will do, the one that doesn’t put unnecessary stress on your joints, especially if they’re hurting, and the one that will challenge both your muscles and your cardiovascular system and allow you to break a good sweat.